1. Field of the Invention
This invention relates to fluid samplers and particularly to subsurface samplers used in petroleum exploration.
2. Discussion of the Related Art
In petroleum exploration, it is often desirable to sample fluids within a borehole at particular stages of the drilling, testing or production processes. This is particularly the case when the well bore has entered or passed through a zone of interest within the earth. The fluid samples provide valuable information to the reservoir engineer as to the viability of the well being drilled.
The current design of subsurface samplers have not changed drastically over the past two decades. In general, a typical subsurface sampler consists of a predetermined length of steel tubing having a spring-actuated valve or stopper in each end. Each valve opens inwardly and when closed, forms a tight seal. Both valves are interconnected by a collet mechanism within the sampler tube so that the manual closing of one valve causes the other valve to close. When released, each valve snaps closed driven by the spring on each end.
A motorized triggering mechanism is threaded to the upper end of the sampler tube and is electrically connected via a wireline to a power source at the surface. The electrical trigger holds the valves open until an operator at the surface directs the trigger to release.
It used to be that a single sampler tube was lowered into a borehole to a preselected depth to collect a fluid sample. Because of ever-increasing drilling depths and multiple pay zones, a plurality of samples required a long period of time to complete.
Leutert (North Sea) Ltd. of Aberdeen, Scotland, manufactures a device used to couple two or more of the previously described subsurface samplers in tandem. The coupling device provides a mechanical triggering mechanism dependent upon the upper sampler to close the lower sampler. Reportedly, the Leutert trigger uses the kinetic energy created by the closing action of the lower valve in the upper sampler to drive a retaining ring from around a multi-finger collet. The collet is allowed to open, thereby allowing a rod coupled to the upper valve in the tandemly-coupled, lower sampler to move upwards, thus closing the valves therein. The closing action of the lower valve in the lower sample may in turn drive a second Leutert trigger, actuating a third sampler, and so on.
A major disadvantage of the Leutert triggering device is that it often times does not work. The outward force of the collet upon the retaining ring creates large amounts of friction thus preventing the retaining ring from being moved by the closing of the lower valve in the upper sampler.
A second disadvantage in the Leutert triggering device is a spring which often times prevents the retaining ring from being moved off the collet and thus prevents the collet from opening.
Another disadvantage in the Leutert triggering device is the mechanical complexity of the device which results in increased manufacturing and operating costs.